There are different motor sets, which a human subject can be in or act
from: He or she can be in a self-initiated voluntary movement set (action)
or in a response set (re-action). Also, imagery sets are available that
are necessary for the acquisition and practice of skill. Most important
are such imagery sets for rehearsal in theatre, dance, music, sports, combat,
etc.

When a human subject performs a voluntary movement, cerebral DC-waves
can be recorded that precede the movement and most probably are an expression
of the brain preparing for the execution of the voluntary movement. Our
group has been interested for the last 32 years now in slow potential shifts
preceding such movement [28,29]. The negative shift occuring under these
circumstances is called the Bereitschaftspotential (BP) or readiness potential
and starts as early as 1 to 2 s prior to the onset of EMG activity in the
muscle. According to theory of EEG, negativity has been related to activity
of the cortical area under study, while positivity has been related to
inactivity. The self-initiated voluntary movement set is experimentally
realized in the Bereitschaftspotential paradigm. The BP paradigm investigates
internally produced movements. Libet calls them `endogenous' [39], although
he conceives the initiation of voluntary movement differently from us:
In Libet´s theory, the cerebral initiation of a voluntary act starts
on an unconscious level which only later leads to the appearance of conscious
intention. Such thinking makes him bypass the assumption of free will in
the initiation of voluntary movement. However, if we start our considerations
from a basic categorical aspect, we have to conceive at least two fundamentally
different categories of movement:

Movements can be internally initiated (self-initiated) or externally
initiated (triggered by events from the outer world). In the first case
we speak of actions, which we perform in the absence of an external
cue - in our opinion out of our free will; in the latter case we speak
of re-actions upon stimuli from the environment. The experimental
paradigm investigating the first category of movements is, as mentioned
above, the BP paradigm (self-initiated internally cued 'endogenous' volitional
acts [3-5,9].

2. Externally triggered movement set (re-actions)

The experimental paradigm for the second category of movements is the
CNV paradigm. Walter et al., introduced this classic reaction time experiment
into slow cerebral potential rerearch with a warning stimulus S1 and an
imperative stimulus S2 after a second or so which the subject has to re-act
to [50]. Grey Walter´s experiment led to the discovery of another
slow DC-potential shift, which he first called just the 'expectancy wave'.
In the publication, he preferred the less implicating term `contingent
negative variation' (CNV). I think that Grey deliberately selected this
term inviting associations with the classic pavlovian heritage of the contingent
reflex in animal conditioning. Thus, the CNV paradigm is the prototype
for movements that are not willful in the strict sense, since they do not
require will for initiating them; the starting command comes from the outer
world. To be precise, the CNV setting may also require will - e.g. in a
choice reaction time experiment, the decision making process undoubtedly
needs will - however, will in decision, not in initiation. Will in initiation,
so to say `will in the time domain', is required in the BP paradigm. In
the CNV paradigm, the individual does not need to care about `when to do'
(i.e. about the right moment to start the movement), the stimulus from
the outer world gives this command. The `when to do' is the last of three
questions, and we will now explain, why we attribute this question to the
supplementary motor area (SMA, [6]).

3. What to do? How to do? When to do?

In more detail, our experimental data so far suggest that the frontal
cortex has important functions in central voluntary movement physiology,
in particular the mesial frontal surface including the SMA - maybe `supplementary'
is more important than `primary' (MI) - these include: (3.1) decision about
the right moment to initiate a voluntary act, when to do; (3.2) temporal
coordination of movements, sequences of movements, bimanual and other interlimb
coordination [11,33,36]; (3.3) participation in motor learning [34], also
cf. lesion of an SMA [1,10,34].

3.1 Decision about the right moment to initiate a voluntary movement

If we are dealing with actions (BP) - as opposed to re-actions (CNV)
- , which a subject initiates out of his or her own will, then we have
to postulate a center in the brain that - in the absence of a trigger from
the outer world - starts the movement. Our real volitional actions, self-initiated,
endogenous, performed at our free will, must have a 'pacemaker', trigger,
go-signal, motor command, starting sign or however we name the initiation
mechanism that launches the movement. Kornhuber and we suggest that it
is the SMA that fulfils this priming function [6,8,12,30]. Kornhuber has
put forward the hypothesis that a voluntary action needs three questions
to be answered: (1) What to do, (2) How to do, and (3) When
to do. All three strategic decisions are thaught to be carried out by the
frontal cortex - Eccles´ neo-neocortex - since they are abandoned
by frontal lesions, albeit in different frontal regions [15,16].

A tripartition of the frontal cortex has been suggested regarding these
three fundamental questions of volition [8]. The frontoorbital cortex
is probably primarily involved in deciding 'what to do.' At least lesions
of these structures render the patient unable to decide what to do, particularly
what is right to do, what is appropriate to do [27]. The 'How question'
is primarily dealt with by the frontolateral cortex with its strong
corticocortical connections with the sensory association areas of the parietal
lobes. Quick decisions regarding the tactics, the 'how' (i.e. what is the
best way) to achieve the goal requires always the newest information about
the sensory situation. These areas include the prefrontal and premotor
areas of the frontal convexity.

After the 'what to do' and the 'how to do' questions are solved, all
what is left to be decided is the 'when to do', i.e. to decide about the
right moment to start the action. This is the task of the frontomesial
cortex, including the SMA. The 'when to do' is the final question in
the motivational chain and is as close to the start of the movement - and
time-locked to it - that it can be recorded by the BP paradigm, while the
other 2 decisions 'what to do' and 'how to do' cannot be directly investigated
by our experimental paradigm.

It was one of our early observations that the BP is composed of two
components [4,5,32]. The early component (BP1) has a vertex maximum, is
bilateral, almost bilaterally-symmetrical, and its slope is usually of
a low to moderate steepness. The late component (BP2) has a C1 or C3 maximum
(with right-sided actions), is asymmetric (contralaterally dominant), and
its slope usually has a higher steepness. These characteristic differences
in topography made us suggest that the symmetric BP1 is principally generated
by the frontocentral midline including the SMA (with always the two SMAs
being active even preceding unilateral actions), while the asymmetric BP2
is principally generated by the MI.

In the MEG (magnetoencephalography), the two components can be seen
as well (Bereitschaftsfeld (BF) 1 and 2, [7,14]. In order to overcome the
problem of cancellation of the opposing SMA dipoles, experiments were carried
out in a patient having a lesion in his right SMA due to anterior cerebral
artery infarction [37]. The results clearly show (with right-sided movements)
that the only remaining left SMA creates a well-defined dipole during the
early BF period (BF1, 1200 to 600 ms prior to the onset of movement), while
during the late period (BF2, 200 to 0 ms prior to the onset of movement)
the principal generator is a dipole in the left MI (area 4 hand area).
This is one of the experimental supports for our hypothesis that SMA leads
MI in time in the final motivational cascade prior to the execution of
the motor act. Other supports are that in our BP experiments it is always
the SMA (midline) that shows the earliest activity prior to starting the
motor act. Only later comes the MI activity into play [36.

Thus, we think that in the pre-movement motivational cascade when it
comes to intention or the channeling of motivation into execution of movement,
it is the frontocentral mesial cortex including the SMA that is involved
in this function. Our critics often say that we are attributing a 'supramotor'
function to the SMA, but that is not true. We attribute a premotor function
to the SMA, and this, indeed, is the SMA´s essential function: SMA
is pre-motor with respect to the motor cortex, i.e. it is upstream of MI
in the final pathway. Or in other words, we extend the premotor nature
the SMA undisputedly has in brain topography, cytoarchitecture, hodology
and developmental systematics also to physiology: it is pre-motor also
in time. In this respect it might be interesting to note that cooling the
SMA in monkey abolishes the execution of a motor task requiring a premovement
selection process [46,47]. In all our measurements of the onset time of
the BP, its earliest onset time is always in recordings from the vertex
(Cz) or slightly before the vertex (FCz, [3-5,9]).

3.2 Temporal coordination of movements, sequences of movements, bimanual
and other interlimb coordination.

Lang et al. [35] found an activation of the SMA in sequential motor
tasks. The sequences were comprised of different flexions or extensions
of the two index fingers and hands. At variance were the combinations how
the above elements had to be composed to sequences by the subject. The
DC recordings of the movement-related potentials showed `phasic' components
immediately prior to and after the onset of every single movement element
of the sequence. These, however, were superimposed upon `tonic' DC potential
shifts (BP prior to and negativity of performance `N-P' during the movement).
This tonic component (shift of the cortical steady potential) - referred
to baseline before the onset of the movement sequence - can be positive
or negative. The level of negativity (which is activity) describes the
`activation background' out of which the single elements of the sequence
evolve. This negative DC background varied to a large extent with type
and complexity of the sequence and the coordinative demands of the task,
being significantly more negative over the frontocentral midline in the
complex task as compared to the simple task. More precisely, it was over
the frontocentral midline (SMA) that in the complex task as compared to
the simple one both the negativity and the regional density of the inward-directed
current flow (Current Source Density, CSD, [40]) significantly increased,
while over the MI hand area of both sides, there was no difference regarding
task complexity. The side of the performing hand influenced negativity
and CSD of the MI areas significantly but not the frontocentral midline
(SMA). This double dissociation clearly shows the existence of two spatially
separate neuronal systems with different functionality. This is important
to note because it renders Bötzel et al.´s [2] assertion unlikely,
who failed to find any SMA contribution to the BP, when applying spatio-temporal
dipole source analysis (STDSA) techniques. These authors along with Toro
et al. [48] thus created a discrepancy not only with our findings but also
with Ikeda et al. [26] and with Rektor et al. [45], who both - using intracranial
(the latter even intracerebral) recordings - proved an SMA contribution
to the BP. The explanation is that STDSA is not sufficient to resolve the
two principal BP sources (SMA and MI) so that they are lumped together
in Bötzel et al.´s [2] analysis (cf. [43]).

It is of interest to investigate the cerebral potentials accompanying
the performance of voluntary movement sequences over a longer period. Lang
et al [38] investigated changes of cortical activity when executing learned
motor sequences over an epoch of 20 s. The quality of performance remained
constant over the epoch and the duration of the entire experiment. However,
the topographies of the DC potentials changed considerably: in parietal
recordings the potential remained stable over the 20 s epoch, over the
contralateral MI it declined only slighly. However, over the frontocentral
midline (SMA) it declined significanty, reaching baseline at the end of
the epoch. This may have to do with a transition from consciously-controlled
to more automatic movement execution and it appears that the SMA activity
is not immediately associated with processes such as the programming and
execution of movement but rather with a supervising control of these
processes. Thus, SMA activity is not only necessary for the intention and
initiation of the voluntary movement (cf. above) but also for its supervising
control during execution - if necessary, i.e. mainly during the acquisition
of novel tasks. Once the task, albeit temporally complicated, has been
learned to the extent that performance is more automatic, SMA participation
seems to be less and less required. This again shows that SMA is not simply
a `performer' but rather a `supervisor'. Regarding a hierarchy in control
for movement, SMA seems to range over MI also in this respect. However,
some authors think that the proof of this hierarchical concept is still
lacking (cf. [51]). Although also Wiesendanger admits that `in the region
of the medial-frontal cortex there is a gradual change from caudal to rostral
with the posterior portion being more `motor', the anterior more `complex'.
We think this is in good agreement with our thinking and would mean that
our interpretation of SMA function would mainly pertain to the anterior
SMA or pre-SMA of Rizzolatti [41]. In Wiesendanger´s intracortical
motor stimulation experiments, the anterior SMA was `clearly less excitable,
although more sluggish responses may be obtained, dependent on whether
the animal is about to move.' Also the cortico-cortical connections were
more widespread in the anterior SMA [25,51]. Luppino et al. [41] in a detailed
hodological analysis also described fundamental differences in connectivity
between the two SMA portions. Again it would be rather the pre-SMA that
has the properties which we attribute to the SMA on the basis of our experiments.
However, not enough is known yet about the cingulate motor area (CMA).
The CMA is located in the upper bank of the cingulate sulcus, its activation
would produce a radial electrical dipole. Such a dipole is well visible
in the EEG but not in the MEG.

3.3 Participation in motor learning

This SMA function is also well documented by the experiments of Lang
et al. [34]. The SMA is not the only area engaged in motor learning, the
premotor cortex of the convexity is also (electrodes F3 and F4 of the EEG)
as well as the middle frontal gyrus of both sides in SPECT. Basal ganglia
and cerebellum are also involved [13].

It is obviously not easy to define the function of the SMA. Neither
lesion studies in man and monkey nor DC-potential studies in EEG and MEG
in man, nor Emission-CT studies in man nor single unit recordings in monkeys
nor direct cortical recordings in epileptic patients have yielded really
unequivocal results. The reason is that the function of the SMA is so extremely
task-dependent. On the other hand, our hypothesis that SMA - as a prime
function in voluntary self-initiated movement or action - has to do with
intention (for review see [30]) has never been falsified. On the contrary,
piece by piece direct cortical recordings by the Lüders group [26,42]
and others [45] have rather confirmed our notion: while earlier, pre-movement
activity in the SMA had not been found in intracranial recordings, presently
this method - if we accept it as the `gold standard' - has definitely proven
that a BP can in fact be recorded in the SMA [26,45]. Remains the question
as to whether SMA activity leads in time before MI activity. On the basis
of our data this is so in most cases. Let us see whether also this notion
will be confirmed some day.

4. Motor imagery set (mental representation of motor acts)

The `visualisation' of a motor act, i.e. to internally envisage a motor
act without that it actually takes place is the topic of the present special
issue, based on the satellite symposium `Mental representations of motor
acts' on the ENA - European Neurosciences Association 17th Annual
Congress Vienna 4-8 September 1994. This also is a set, a human subject
is capable of - we can see something in our mind´s eye - although
the imaginative power of individuals may be different. This is thought
in psychology or sensory physiology to be an intrinsic personality feature
- that persons are more `eidetic' or less so. In our experiments, we distinguished
`high imagery individuals from low imagery individuals [24]. Psychological
considerations have led to two major hypotheses concerning the neurological
basis of mental imagery. On the one hand it has been said ,,that a subject
is imaging whenever he employs the same cognitive processes that he would
use in perceiving, but when the stimulus input that would normally give
rise to such a perception is absent" (Neisser, cited by Eysenck [18]).
From this it seems likely that imagery activates the same areas of the
brain that are activated by actual perception. This hypothesis is corroborated
by numerous case studies that describe a loss of e.g. visual imagery in
conjunction with visuoperceptive impairment (for review cf. [19]). On the
other hand, imagery has been understood as a nonverbal mode of information
processing that is opposed to verbal processing [44]. Since verbal abilities
are lateralized to the left hemisphere, it has been concluded that imagery
should be a domain of the right hemisphere. However, little empirical support
has been found for this hypothesis (for review cf. [17]), and recently
even the opposite claim of a left hemisphere superiority for imagery has
been made [20,31]. Goldenberg et al. [21-23] found rCBF patterns in SPECT
for visual imagery in the left inferior occipital lobe. In episodic memory
tasks, they found rCBF increase in lateral inferior temporal regions, whereas
in semantic memory tasks, the left inferior parietal lobe was included.

In a SPECT study investigating imagining of colours, faces and a map,
we found that 12 out of 30 subjects reported the spontaneous occurrence
of mental visual images [22,23]. In these subjects flow in both frontoorbital
regions was higher than in those subjects who had not experienced spontaneous
imagery. Voluntary imagery led to an increase of regional flow indices
in basal temporal regions of both hemispheres and to a rightwards shift
of global hemispheric asymmetry. The local changes were distinctly more
marked with faces than with colours or the map. Imagining faces was also
the only condition that led to an increase of activity in the left inferior
occipital region. which has been suggested by previous studies to be a
crucial area for visual imagery. In a companion study using negative cortical
DC-shifts [49] imagery of colours, faces and a map resulted in sustained
negative DC-shifts at temporal, parietal and especially at occipital sites.
The topographic distribution of such DC-shifts was modulated as a function
of whether spatial or visual imagery was performed. During imagining the
spatial map, a parietal maximum was observed, as opposed to a distribution
in favour of temporal and occipital sites during imagining faces and colours.
The results suggested a neuroanatomical dissociation between visual and
spatial imagery. Since a similar visual-spatial dichotomy exists in perception,
the finding was interpreted as further evidence of a shared cerebral substrate
for images and percepts.

The following papers of the satellite symposium `Mental representations
of motor acts' compiled in this special issue are all dealing with the
fact that we can imagine objects, sounds (music), and deliberately motor
acts in our mind´s eye. It is hoped that the present endeavor of
an interdisplinary approach will contribute to our understanding of the
important issue of imagery and particularly motor imagery.